An ion source is a critical component of an ion implanter. An ion source includes an arc chamber housing defining an arc chamber. The arc chamber housing also has an extraction aperture through which a well defined ion beam is extracted by an extraction electrode assembly positioned proximate the extraction aperture. The ion beam passes through the beam line of the ion implanter and is delivered to a target workpiece such as a semiconductor wafer. The ion source is required to generate a stable, well-defined, uniform ion beam for a variety of different ion species. It is also desirable to operate the ion source in a semiconductor production facility for extended periods of time without the need for maintenance or repair.
In operation, a desired dopant gas is provided to the arc chamber of the ion source. The dopant gas is ionized into a plasma by exposing the dopant gas to energetic electrons. The energetic electrons may be generated in a number of ways. One way to generate electrons is to position a filament proximate a cathode as is done with an indirectly heated cathode (IHC) ion source. The filament is generally sheltered from the plasma in the arc chamber to which the relatively massive cathode is exposed. When heated by the filament, the cathode supplies energetic electrons. A well defined ion beam is extracted through the extraction aperture by a biased extraction electrode assembly positioned proximate the extraction aperture.
Over time, undesirable deposits may form on the extraction aperture. The presence and rate of formation of such deposits may be influenced by the selected dopant gas. For some fluorine containing dopant gases such as boron trifluoride (BF3), germanium tetrafluoride (GeF4), and silicon tetrafluoride (SiF4), undesirable deposits such as tungsten may form on the extraction aperture. For other molecular species, the rate of undesirable deposit formation may be even more severe. For example, for carborane (C2B10H12), undesirable deposits such as carbon and boron may quickly form on the extraction aperture. Such deposits on the extraction aperture can block portions of the ion beam extracted there from.
Such undesirable deposits formed on the extraction aperture may adversely affect beam uniformity, beam current, and ion source lifetime. For example, any beam blockage at the extraction aperture may impact the uniformity of the ion beam extracted there from. These non uniformities may then be transmitted and magnified as the ion beam passes down the beam line to the target workpiece. Such magnification may be exacerbated in ion implanters that provide a “ribbon beam” having an approximately rectangular cross section at a workpiece plane defined by a front surface of the target workpiece. The long dimension of the ribbon beam at the workpiece plane may be at least 10 times greater than the long dimension of the beam extracted from the extraction aperture thus magnifying any blockage issues of the ion beam at the extraction aperture. Beam uniformity problems can contribute to dose uniformity problems at the workpiece plane. Dose uniformity requirements continue to become more stringent as some current specifications require dose uniformity of less than 1% variation at the workpiece plane for ribbon beams. An ion beam with poor beam uniformity due to deposits on the extraction aperture may be improved with tuning techniques that reduce beam current levels. In other words, the beam current of the ion beam extracted from the source would be lowered from desired levels to improve beam uniformity. Lowered beam currents can adversely impact throughput of the associated ion implanter or the number of workpieces processed per time period. Finally, ion source lifetime may be limited by undesirable deposits on the extraction aperture.
Some conventional techniques for removing undesirable deposits in an ion source include introducing particular cleaning gases such as reactive gases into the arc chamber. Although largely effective, such cleaning gas techniques may take 10-60 minutes to complete and, of course, require the cleaning gas and flow control of the same. In addition, the number of molecules in the cleaning gas must be at least as big as the number of atoms to be cleaned. For example, two hours of accumulated deposits might be cleaned in 10 minutes only with a very high flow rate of the cleaning gas. Therefore, the length of time to complete such a cleaning operation adversely impacts throughput.
Accordingly, it would be desirable to provide a technique for cleaning an extraction aperture of an ion source which overcomes the above-described inadequacies and shortcomings.